1. Field of the Invention
The invention relates to amplifier circuits, and more particularly to dynamic range improvement for amplifier circuits.
2. Description of the Related Art
A radio frequency receiver system receives and processes a radio frequency signal. An antenna receives the radio frequency signal for the radio frequency receiver system. The received signal generated by the antenna comprises a wanted signal component and a blocker signal component. The wanted signal component is generated by a target transmitting station, and the blocker signal component, which is also referred to as a jammer signal component, is generated by jamming stations. Ordinarily, the wanted signal component has a small amplitude (e.g. −99 dBm) in comparison with the amplitude of the blocker signal component (e.g. 0 dBm).
The blocker signal received by the antenna induces difficulties in the processing of the wanted signal. There are three main problems when a large block beside the wanted signal is received. The first one is wanted signal gain compression. When the amplitude of the blocker signal is too large such that the voltage headroom of an amplifier is not enough to process the blocker signal, the wanted signal gain is compressed; so the signal to noise ratio (SNR) is also degraded. The second problem to degrade the SNR is the reciprocal mixing noise. The received blocker signal is always mixed with the phase noise of the local oscillation (LO) signal and it results in extra reciprocal in-band noise. The third one is in-band blocker induced noise. Due to finite linearity of the receiving chain, the device noise is also mixed with the blocker signal and some of them fall into the signal band and degrades SNR.
A designer of the radio frequency receiver system therefore must increase the dynamic range of the radio frequency receiver system to improve the performance thereof. The dynamic range of a system indicates a ratio of the largest tolerable blocker (e.g. 0 dBm) to a given wanted signal (e.g. −99 dBm). To increase the dynamic range of a radio frequency receiver system, a filter for filtering out the blocker signal component from the received signal is added to the radio frequency receiver system. Referring to FIG. 1, a block diagram of a portion of a radio frequency receiver system is shown. In one embodiment, the radio frequency receiver 100 comprises an antenna 102, a SAW filter 104, an LNA 106, a mixer 108, a baseband amplifier 110, and an ADC. The antenna 102 receives a radio frequency signal to generate an input signal. The SAW filter 104 filters out the blocker signal component from the input signal. Therefore, the receiving chain after the SAW filter 104 does not affect by the blocker signal and the dynamic range is improved. However, the SAW filter 104 degrades the wanted signal level; so the receiving sensitivity is also degraded. Moreover, it increases the bill of material (BOM) cost.
If the SAW filter 104 is removed in the receiving system, the blocker signal is directly received by LNA 106. Referring to FIG. 2A, a circuit diagram of a conventional low noise amplifier 200A to handle such blocker signal is shown. In FIG. 2A, transistors M1 and M2 are biased in the weakly inversion region. Therefore, when a large blocker signal is received, the low noise amplifier 200A functions like a class-AB amplifier. Such class-AB LNA has self-biasing characteristic as a general class-AB amplifier. The current consumption of the class-AB LNA is automatically adjusted according the blocker level. When the received blocker signal is larger its current consumption also becomes larger and this results in larger gain, i.e. gain expansion. Such gain expansion can compensate the gain compression from other nodes in the amplifier, e.g. the output nodes of the amplifier. Therefore, the dynamic range can be enhanced without boosting its average current.
Although the dynamic range and current consumption problem of the input stage (i.e., transistors M1 and M2) are solved, a load inductor Lload is required to increase its output voltage headroom to prevent the dynamic range is limited by it. However, the load inductors Lload occupy a large chip area and increases a hardware cost. When the load inductors Lload are replaced with PMOS transistors (as shown in FIG. 2B (i.e., the “conventional class-AB amplifier” in the detailed description)) to reduce a chip area, the voltage swing level of the output signal of the low noise amplifier 200B is reduced, and the low noise amplifier 200B has difficulties to process an input signal comprising a blocker signal component with high amplitude. The situation is worse in the deep submicron technology since the supply voltage is lower. Thus, an amplifier circuit is required in a radio frequency receiver system for increasing the dynamic range of the radio frequency receiver system.